This invention relates to systems for curing polymers or other curable materials. More specifically, this invention relates to unique systems for curing various materials, particularly polymers by contacting them with a microporous film charged with a curing agent.
Polymer curing is a technique which is well-known and understood in the art. Basically, it may be said that polymer curing is the phenomenon of converting a generally two-dimensional or non-crosslinked polymer chain into a three dimensional or crosslinked polymer chain or molecule. The crosslinking or curing of an uncured polymer affects many of its basic properties, such as strength, toughness, heat resistance, extrudability, moldability, flexibility, adhesiveness, chemical resistance, dielectric characteristics, and the like. Thus, the phenomenon of curing is an extremely important industrial technique.
The curing of any given polymer is usually achieved by one of two basic techniques. The first technique is usually referred to as a thermal or heat-curing process. As the name implies, this technique provides for simply heating a polymer system, whereupon, due to the selection of initial ingredients, a polymer chain of the system crosslinks with other polymer chains in the system to effect a three-dimensional molecule or polymer.
The second technique requires the addition of a curing agent to the system. This curing agent may be in the form of a coupling or additive agent, i.e., where the agent itself forms a part of the cross-linkage, or it may merely be a catalyst (or accelerator) which initiates an auto-crosslinking among the two-dimensional chains. Very often the above two techniques are combined so as to achieve an even more favorable curing process. The ability to cure particular polymers has opened wide vistas for the polymer chemist and the polymer industry as a whole since it has enabled polymers to be effectively used in such widely diversified fields as adhesives, protective coatings, reinforcements, molded articles, extruded articles, casting and plastic tooling, thermal insulation, chemical insulation, and the like.
Of special interest to the polymer industry had been the relatively recent development of certain polymers known as "epoxy resins." Epoxy resins are now well-known in the industry and are commonly formed by reacting epichlorohydrin with bisphenol A (4,4-isopropylidene diphenol). Recently other hydrins and polyols have been developed and used as substitutes for the above ingredients to obtain various selected properties. For example, various aliphatic glycols and novolac resins may be used in place of the bisphenol. The use of novolac resins generally increases the heat resistance of an epoxy resin. Furthermore, various additives and modifiers have been developed and may be added to the basic epoxy resins to vary their properties. Such additives and modifiers are well-known in the art. For example, epoxy-phenolic resin systems exhibit extreme hardness and chemical resistance while blends of epoxy resins with various nylons exhibit extremely good shear-strength characteristics.
It may generally be stated that epoxy resins are curable materials which achieve their best characteristics and thus are most useful when cured. Although certain of these resins can theoretically be cured by the addition of heat alone, the most useful epoxy resins require the addition of a curing agent either with or without heat to effect a useful amount of crosslinking. The curing agents used are well-known in the art and may be divided into four basic groups: (1) amine type, (2) acidic type, (3) aldehyde condensation products, and (4) Lewis acid catalysts. Examples of amine type curing agents are aliphatic and aromatic amines, polyamides, tertiary amines, and amine adducts. Acid type curing agents include both acids and acid anhydrides, while the aldehyde condensation products generally envisioned are the phenol-, urea-, and melamineformaldehyde resins. The Lewis acid catalysts usually take the form of complexes such as the complex of boron trifluoride with various amines such as piperidine or monoethylamine.
Of particular interest in the epoxy resin industry has been the ability of various epoxy resins and epoxy resin systems to form extremely strong adhesive bonds upon curing. The adhesive properties of these resins when cured have been found so good that they have been used in metal-to-metal, glass-to-glass, and wood-to-wood bonding as well as in printed circuits and body solders. In some instances, epoxy adhesives have replaced brazing and soldering in the metal-to-metal bonding area.
Acceptability of epoxy resins in the adhesive art stems from various unique properties exhibited by these resins. For example, they are stronger than phenolic resins; they are virtually 100 percent reactive with no volatiles evolved during cure; they have excellent flow characteristics and require only slight pressure to force sufficient adhering contact; and they are suitable for a wide variety of environments since a great number of curing agents are available for them and a great number of modifiers are compatible with them.
One of the drawbacks to epoxy resin adhesive systems which has heretofore troubled the art, is the need for a "two-package" system. That is to say, heretofore, in order to bond two substrates together, a curing agent and an uncured epoxy resin system, each provided in separate containers, had to be mixed just prior to bonding and applied as a mixture to the interface of the surfaces of be bonded. The need to premix and apply the resin curing agent system before significant curing or cross-linking has occurred is a definite detriment since it removes a degree of flexibility from the bonding process. Such a detriment is especially acute when using the commercially desirable epoxy resin systems which cure rapidly.
Microporous films containing a plurality of discrete open voids are generally well-known in the art. Although these films may be made from a variety of well-known materials, they are generally made from natural or synthetic polymeric materials. The voids formed therein have generally been used for various purposes such as to lend opacity to the film, to hold a printing dye or dye intermediate for manifold use, to effect a slow release of perfumes, to act as an ion membrane fuel cell or to act as a semi-permeable coating.
One known method of making microporous open void films as above described is to dissolve a thermoplastic, water-insoluble polymer and a linear polypolar polymer into a single common polar organic solvent. The solution is then applied as a film to a substrate and contacted with water for a period of time sufficient to displace the organic solvent from the film. After treatment with water the film is dried until substantially all the water is removed. The resulting film is microporous and contains a plurality of discrete, open voids capable of being charged with a variety of substances.
Another known method of making microprous films is to prepare an aqueous dispersion of a polymer and a water-soluble organic solvent for the polymer which boils above 100.degree. C. and which is present in a concentration that is insufficient to dissolve the polymer. This dispersed mixture is then coated on a substrate as a film and a substantial amount of the water is removed by evaporation below 100.degree. C. until partial coalescence of the polymer occurs as indicated by substantial clarification with tackiness. This tacky film is then washed with water or another liquid in which the polymer is insoluble but which dissolves the solvent for the polymer, to produce a coherent film substantially free from dissolved polymer and organic solvent. After this wash with water, etc., the film is dried at a temperature below its softening point. Such a film is found to contain a plurality of discrete open voids.
Although the above-disclosed prior art techniques do provide microporous films, superior microporous films containing a plurality of discrete open voids and capable of being charged with various materials may be made by the technique disclosed in Applicant's U.S. Pat. No. 3,544,489, the entire disclosure of which is incorporated herein by reference. Basically, this technique comprises forming a film of the composition comprising (a) a thermosetting resin and (b) a solvent-extractable thermoplastic resin, which resins are at least partially compatible, and subsequently curing the thermosetting resin, such as by heating. Upon curing of the thermosetting resin, the thermoplastic resin forms minute, discrete particles in the thermoset resin matrix. The thermoplastic resin is then extracted from the film such as by means of a suitable solvent for the thermoplastic. There is thus obtained an opaque chargeable film of the thermoset resin which is substantially continuous and contains a large number of discrete, open voids.